Basic concepts of Radiation Therapy Flashcards
cancer in pet animals
Common, increasing prevalence
Every 2nd dog is euthanized due to „cancer“
Radiation therapy: second most used treatment modality in man
Model for research
resection “en bloc”
can’t remove just the tumour as tumour cells remain in the surrounding tissue
roles of radiation therapy
INCOMPLETELY RESECTED TUMORS (treatment of choice) Non resectable tumors (results depending of tumor type) Pain control (Bone cancer/bone mets)
ionising radiation
Direct damage
Indirect damage (free radicals)
Unit for RT: Grey
reproductive death
DNA damaged - cell dies or lives but is unable to divide further
damage to DNA by ionising radiation effects
Mitosis delayed (10-100Gy)
Radiation damaged cells usually die after 1 or 2
attempts at mitosis
Tissues with high proliferation rate express radiation damage relatively quickly
most radiation sensitive cells
undifferentiated high mitotic rate bone marrow intestinal crypt cells germinal layer of epidermis tumours Tumoral mecanisms of DNA repair not as efficient as in normal cells
fractionation
The total dose of radiation used to treat a tumour is
usually divided into a number of fractions
spares normal tissues because it allows repair of sublethal damage and repopulation
increases damage to tumour because of reoxygenation + redistribution of cells into radiosensitive phases of cell cycle
4 R’s of radiation therapy
repair
repopulation
reoxygenation
redistribution
repair
tumor cells less able to repair DNA damage (Tumor cells are often oxygen + nutrient deficient)
repopulation
RT will often stimulate cell division.
Cells may respond to the death of adjacent cells by “accelerated repopulation”
In normal tissue this is kept under control by normal homeostatic mechanisms, but this is not true for the tumour.
need to give another dose to counteract repopulation + kill the cells
reoxygenation
The tumor contains a mix of aerated and hypoxic cells.
Immediately after RT, most cells in the tumor are hypoxic, but they tend to return at the pre-RT pattern because of reoxygenation
redistribution
Cells are more sensitive to RT in some phases: M-G2-G1-ES-LS
The remaining cells will be “synchronized” + eventually will move into a more sensitive phase
this is when to give next RT fraction
types of radiation used for radiotherapy
X-rays - Orthovoltage, 120-300kVp (not used much), Megavoltage, >1MeV “Linear accelerator”
Gamma rays - Cobalt-60, 1.2MeV
Particles - particles (electrons) in radioisotopes - Strontium-90: plesiotherapy, Iridium-192: brachytherapy
treatment machines
Cobalt-60-therapy unit
Linear accelerator
Cobalt-60-therapy unit
Radioactive source
Photons
Fixed energy (1.24 MV)
Technical requirements ↓
linear accelerator
Variable energies electrons / photons No radioactivity High technical maintenance High accuracy
electron beam
Various energies possible (e.g. 5-16 MeV)
Rapid dose reduction (depending on energy)
Therapeutically useful depth: 1.5-5.6 cm
Single fields, simple dose calculation
photon beam
High penetration Slow dose reduction Field arrangements necessary Penetration of normal tissue CT-based treatment planning
goals of radiation therapy
curative/definitive (long term tumour control)
palliative (palliation, stabilisation of disease, pain reduction)
curative radiation therapy
Absolute Indications - RT shows better results than other therapies
Relative Indications - RT shows same tumor control, but other advantages (functional, cosmetic)
Combination therapy - RT + surgery +/- chemo
primary Tx modality
Brain tumor, tumor of head and neck (oral, nasal), MCT, epulis
post Sx adjuvant RT
Mast cell tumor, Soft tissue sarcoma, Feline vaccine-associated sarcoma, etc.
nasal tumours
2/3 carcinomas; 1/3 sarcomas median survival without treatment ~ 3 months surgery alone ~ 3-6 months radiation therapy alone: ~ 8-20 months RT + SX: 47 months
